Heart failure (“HF”) refers to an increasingly ineffective heart, or a dysfunction in the pumping action of the heart due to the heart's inability to contract or relax properly. Typically, the lower chambers of the heart (ventricles) do not beat as efficiently as they do in a healthy heart. As heart failure progresses, the body attempts to compensate for it. In an attempt to improve pumping function, the heart increases its muscle mass in a process called remodeling. While the remodeled, enlarged heart does pump more blood initially, it eventually requires more oxygen than it can get, resulting in further damage. In addition, the walls of the remodeled heart stiffen, and pumping efficiency decreases.
HF can be caused by an abnormality in systolic function leading to a defect in the expulsion of blood, which is known as systolic HF; or by an abnormality in diastolic function leading to a defect in ventricular filling, known as diastolic HF (“DHF”). Diastole is the normal rhythmic relaxation and dilatation of the ventricles during which they fill with blood. Atrial contraction occurs near the end of diastole to assist ventricular filling. Systole is the rhythmic contraction of the heart, especially of the ventricles, by which blood is driven through the aorta and the pulmonary artery. The preload is the load present before contraction of the ventricles begins, and is provided by the venous return that fills the ventricle during diastole. In DHF, ventricular stiffness or reduced compliance leads to limitations on the use of preload reserve because of rapid increases in cardiac filling pressures at normal or slightly increased cardiac volume. These effects limit cardiac output and cause dyspnea during exercise. See, e.g., Eduardo B. Saad, Cardiology: Diastolic Heart Failure, http://www.medstudents.com.br/cardio/cardio5.htm, 2000.
Detecting and monitoring the progression of DHF is difficult. Preserved ejection fraction (“EF”) might appear to be a good indicator of DHF. However, patients with DHF generally have a preserved ejection fraction (“EF”) greater than 50%, which is considered to be in the range of normal cardiac condition. Hence, EF alone is not sufficient for determining DHF. Recent findings suggest that changes in end-systolic elastance, effective arterial elastance, and pressure-volume curve are good indicators of DHF. See, e.g., Miho Kawaguchi, Ilan Hay, Barry Fetics, and David A. Kass, Combined Ventricular Systolic and Arterial Stiffening in Patients With Heart Failure and Preserved Ejection Fraction: Implications for Systolic and Diastolic Reserve Limitations, Circulation, February 2003; 107: 714-720. However, obtaining these values using conventional techniques is difficult, often requiring complex procedures with special sensors and tools.
Treatments for HF include medications, surgical procedures, heart transplant, and electrical stimulation therapy. Various electrical stimulation therapies are practiced by implantable cardiac stimulation devices, some of which are capable of pacing the heart of a patient in single or dual chamber modalities. Some heart failure patients may be candidates for an implantable cardiac stimulation device to provide cardiac resynchronization therapy (“CRT”). CRT involves pacing in three chambers instead of one chamber pacing in the right ventricle or two chamber pacing in the right ventricle and right atrium. A CRT device paces the right atrium, the right ventricle, and the left ventricle. The goal of CRT therapy is to synchronize the right and left ventricles to improve the efficiency of the contraction. The CRT functionality can be provided in pacemakers and implantable cardioverter defibrillators (“ICDs”) which have built-in pacemakers. Although conventional implantable cardiac devices such as pacemakers and ICDs monitor various parameters, they are unsuitable for determining DHF.
To make electrical stimulation therapy and other treatments of HF more effective, improvements in monitoring the progression of DHF and other HF conditions using implantable cardiac devices are desirable.